The present invention relates generally to antennae comprising a plurality meanderlines (also referred to as variable impedance transmission lines or slow wave transmissions lines), and specifically to such an antenna providing multi-band operation using a simple or complex polygonal or irregularly shaped radiating element or a plurality of such radiating elements.
It is generally known that antenna performance is dependent upon the antenna shape, the relationship between the antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna parameters, including input impedance, gain, directivity and the radiation pattern shape. Generally, the minimum physical antenna dimension must be on the order of a quarter-wavelength of the operating frequency, thereby allowing the antenna to be excited easily and to operate at or near its resonant frequency, which in turn limits the energy dissipated in resistive losses and maximizes the antenna gain.
The burgeoning growth of wireless communications devices and systems has created a significant need for physically smaller, less obtrusive, and more efficient antennae that are capable of operation in multiple frequency bands and/or in multiple modes (i.e., having different radiation patterns). As is known to those skilled in the art, there is an inverse relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennae. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennae capable of efficient multi-frequency and/or wide bandwidth operation. Finally, it is known that the relationship between the antenna frequency and the antenna length (in wavelengths) determines the antenna gain. That is, the antenna gain is constant for all quarter-wavelength antennae (i.e., at that operating frequency where the antenna length is a quarter of a wavelength).
One prior art technique that addresses some of these antenna requirements is the so-called xe2x80x9cYagi-Udaxe2x80x9d antenna, which has been successfully used for many years in applications such as the reception of television signals and point-to-point communications. The Yagi-Uda antenna can be designed with high gain (or directivity) and a low voltage-standing-wave ratio (i.e., low losses) throughout a narrow band of contiguous frequencies. It is also possible to operate the Yagi-Uda antenna in more than one frequency band, provided that each band is relatively narrow and that the mean frequency of any one band is not a multiple of the mean frequency of another band. That is, a Yagi-Uda antenna for operation at multiple frequencies can be constructed so long as the operational frequencies are not harmonically related.
Specifically, in the Yagi-Uda antenna, there is a single element driven from a source of electromagnetic radio frequency (RF) radiation. That driven element is typically a half-wave dipole antenna. In addition to the half-wave dipole element, the antenna has certain parasitic elements, including a reflector element on one side of the dipole and a plurality of director elements on the other side of the dipole. The director elements are usually disposed in a spaced-apart relationship in the transmitting direction or, in accordance with the antenna reciprocity theorem, in the receiving direction. The reflector element is disposed on the side of the dipole opposite from the array of director elements. Certain improvements in the Yagi-Uda antenna are set forth in U.S. Pat. No. 2,688,083 (disclosing a Yagi-Uda antenna configuration to achieve coverage of two relatively narrow non-contiguous frequency bands), and U.S. Pat. No. 5,061,944 (disclosing the use of a full or partial cylinder partially enveloping the dipole element).
U.S. Pat. No. 6,025,811 discloses an invention directed to a dipole array antenna having two dipole radiating elements. The first element is a driven dipole of a predetermined length and the second element is an unfed dipole of a different length, but closely spaced from the driven dipole and excited by near-field coupling. This antenna provides improved performance characteristics at higher microwave frequencies.
One basic antenna model commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. The personal communications (PCS) band of frequencies extends from 1710 to from 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz and 2.68 inches long at 2200 MHz, and has a typical gain of a 2.15 dBi. A derivative of the half-wavelength dipole is the quarter-wavelength monopole antenna located above a ground plane. The physical antenna length is a quarter-wavelength, but the ground plane changes the antenna characteristics to resemble a half-wavelength dipole. Thus, the radiation pattern for such a monopole is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above a ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern (along the radial axis) with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics. Finally, another conventional antenna is the patch, which provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter- or half-wavelength antenna, the patch antenna has a low radiation efficiency.
The present invention discloses an antenna comprising one or more conductive elements, including a horizontal element and at least two oppositely disposed vertical elements, each connected to the horizontal element by a meanderline coupler, and a ground plane. The meanderline coupler has an effective electrical length through the dielectric medium that influences the overall effective electrical length, operating characteristics, and pattern shape of the antenna. Further, the use of multiple vertical elements or multiple meanderline couplers on a single vertical element provides controllable operation in multiple frequency bands. An antenna comprising meanderline couplers has a smaller physical size, yet exhibits enhanced performance over a conventional dipole. Further, the operational bandwidth is greater than typically available with a patch antenna. Finally, an antenna constructed with two meanderline couplers and more than one horizontal element offers polarization diversity depending on the relationship between the transmitted/received signal and the orientation of the radiating/receiving elements.
A meanderline coupled antenna constructed according to the prior art typically operates in two frequency bands, with a unique antenna pattern for each band (i.e., in one band the antenna has an omnidirectional donut radiation pattern (referred to herein as monopole mode) and in the other band the majority of the radiation is emitted in a hemispherical elevation pattern (referred to as loop mode). According to the teachings of the present invention, the antenna comprises a plurality of horizontal conductors (also referred to as top plates) or a single horizontal conductor with an shape determined by the desired antenna characteristics. The multiple top plates or the shaped top plate provides multiple resonant frequencies or multiple resonant frequency bands and therefore the antenna operates in multiple modes in a single frequency band, dependent upon which one or more of the multiple top plates are excited or in the shaped top plate embodiment, dependent upon the particular segment or region of the shaped top plate that is excited.